1) Field of the Invention
The present invention relates to an apparatus and a method for discharge surface treatment. More specifically, this invention relates to the power supply apparatus for discharge surface treatment which uses a green compact electrode as a discharge electrode, and allows a pulse-type discharge to take place between the discharge electrode and a workpiece so as to form a film, which film is made of an electrode material or a material obtained when the electrode material reacts to the discharge energy, on a surface of the workpiece.
2) Description of the Related Art
FIG. 7 shows a prior discharge coating apparatus disclosed in Japanese Patent Application Laid-Open No. 54-153743. The discharge coating apparatus has a working tank 1 for housing working fluid, an electrode (covered electrode) 2 which is arranged so as to face a workpiece W in the working tank 1 with a predetermined discharge gap therebetween. A power supply apparatus (pulse power supply apparatus) 3 applies a pulse-like voltage to between the workpiece W and the electrode 2.
When the pulse-like voltage is applied to between the electrode 2 and the workpiece W, the discharge surface treatment by means of the discharge coating apparatus allows pulse-type discharge to take place between the electrode 2 and the workpiece W. As a result, a film made of the material of the electrode 2 or a material obtained when the material of the electrode reacts to the discharge energy is formed on the surface of the workpiece W.
The power supply apparatus 3 has a DC power supply 4, an oscillator 5 which generates a pulse current of a predetermined frequency by giving a DC current to the oscillator 5 from the DC power supply 4, electric current cut-off means 6 such as a thyristor, and voltage detection means 7 which detects a discharge voltage between the workpiece W and the working electrode 2.
A comparator 8 compares the discharge voltage detected by the voltage detection means 7 with a discharge detection voltage (threshold value Vth) set by a discharge detection voltage setting unit 9. The comparator 8 outputs a forced electric current cut-off command to the electric current cut-off means 6 after constant time .DELTA.t passes from the point of time that the discharge voltage (voltage detected value V) becomes lower than the set value Vth of the discharge detection voltage. The electric current cut-off means 6 forcibly ends the discharge according to the forced electric current cut-off command.
In the discharge coating apparatus having the above structure, the oscillator 5 applies a voltage to between the workpiece W and the electrode 2 that have a predetermined gap therebetween. When the gap between the workpiece W and the electrode 2 attains a predetermined value, discharge takes place between the workpiece W and the electrode 2. The workpiece W is worked by the discharge energy.
When the discharge starts, the inter-electrode voltage abruptly drops at the point of time shown by a point A in FIG. 8. The voltage detection means 7 detects such a drop in the voltage, and after the constant time .DELTA.t passes from the starting of the discharge, the electric current cut-off means 6 cuts off the output of the oscillator 5 so that the discharge is forcibly terminated. After the discharge current completely fails, voltage is again applied to between the workpiece W and the electrode 2 by the output of the oscillator 5.
As a result, long-time pulse is not obtained, and the voltage is cut off at suitable discharge time. Therefore, occurrence of a layer having different properties on the surface of the workpiece is avoided, and a satisfactorily worked surface can be obtained.
At the time of the discharge working, since discharge tailing which generates between the workpiece W and the electrode 2 during the working floats, and thus the resistance between the electrodes is lowered. As a result, the inter-electrode voltage at the time of discharge is also lowered. For this reason, when the set value Vth of the discharge detection voltage is set to a higher value, it is difficult to detect the discharge normally. Therefore, the set value Vth of the discharge detection voltage should be set to a comparatively low value as shown in FIG. 8.
When a green compact electrode obtained by compression-molding metallic powder or metallic compound into an electrode shape is used in the discharge surface treatment, the electrical resistance of the electrode is considerably higher than that of a normal copper electrode. As shown in FIG. 7, the voltage detection means 7 which is connected with a circuit reads also a part of the voltage which drops because of the electrical resistance of the working electrode 2. The characteristic of the voltage detected by the voltage detection means 7 is as shown in FIG. 9, and the detected voltage does not drop sufficiently even after the discharge has terminated so that the discharge cannot be detected.
As a result, the output of the oscillator cannot be cut off suitably, and the discharge with long-time pulse is generated so that it is difficult to maintain the suitable discharge state.
The present invention is devised in order to solve the above problems, and it is an object of the invention to provide a power supply apparatus which cuts off a voltage at suitable discharge time and prevents long-time pulse discharge in a discharge surface treatment using a green compact electrode.